It is generally desirable to reduce power consumption of electronic devices having receiving functionalities. Especially battery-operated mobile terminals benefit from a reduced receiver power consumption. The benefits include longer stand-by and operational times.
Mobile terminals usually conform to one or more mobile communication standards that define, inter alia, operational states for the receiver. As an example, the Long Term Evolution (LTE) standard of the 3rd Generation Partnership Project (3GPP) specifies so-called “idle” and “connected” states for the physical layer of a mobile terminal (also referred to as User Equipment, or UE, in the LTE standard). The physical layer includes receiver components, the operation of which will thus by influenced by the current state setting.
When the UE is in idle state, there are no ongoing transfers in the receiving and transmitting directions. The UE is only waking-up from time to time to check whether a connection request is coming in. An incoming connection request is signalled at so-called paging occasions. In idle state, the power consumption is therefore heavily reduced because the receiver components are switched off most of the time and only briefly switched on at paging occasions.
In connected state, the receiver components are switched on most of the time as the UE has to listen to the Physical Downlink Control Channel (PDCCH), which is transmitted in a first portion of a sub-frame. The PDCCH is used to transfer scheduling grants indicating that there is a transmission on the Physical Downlink Shared Channel (PDSCH) to the UE in a subsequent second portion of the current sub-frame. In case a PDSCH transmission is indicated to the UE either in the PDCCH or by semi-persistant scheduling, the remainder of the sub-frame has to be received and the PDSCH has to be decoded. Reception must also continue in other scenarios such as intra-frequency measurements or Broadcast Channel (BCH) readings.
There are still many scenarios in connected state in which decoding of the first sub-frame portion reveals that the remainder of the sub-frame following the PDCCH is of no interest to the UE and in which reception can be terminated until the next sub-frame arrives. Terminating reception by switching off one or more receiver components during the resulting short gap between the end of the first portion of one sub-frame and the beginning of the next sub-frame is also referred to as micro sleep.
In the LTE standard, a sub-frame has a duration of 1 ms and downlink transmissions are based on Orthogonal Frequency Division Multiplexing (OFDM). OFDM-based systems use block processing that includes a Fast Fourier Transform (FFT) in a digital receiver domain for OFDM demodulation. The digital receiver domain (Digital Front End, or DFE) before the FFT is based on sample processing. The PDCCH can be spread over up to 4 OFDM symbols for a system bandwidth of 1.4 MHz and over up to 3 OFDM symbols for larger bandwidths.
FIG. 1 shows a schematic timing diagram illustrating the processes of entering and leaving a micro sleep mode in an exemplary LTE scenario in which the PDCCH is spread over 3 OFDM symbols. In a regular reception mode (“Rx on period” in FIG. 1), a signal from an analog radio front end is sampled at a given sampling rate and the resulting signal samples are buffered in a memory. The buffered signal samples are subjected to FFT processing for a demodulation of the received OFDM symbols that pertain to the PDCCH. After OFDM demodulation, channel estimation and demapping steps are performed as is generally known in the art.
In a further step, the PDCCH is decoded to determine whether the remainder of the sub-frame has to be received (and whether the PDSCH has to be decoded) also, or whether the receiver can enter a micro sleep mode (“Rx off period” in FIG. 1) in which one or more receiver components are switched off. The time it takes to enter the micro sleep mode (“‘Switching off’ period”) is typically rather short and therefore not illustrated in FIG. 1. On the other hand, the micro sleep mode has to be left early enough (“‘Switching on’ period” in FIG. 1) to ensure that the regular reception mode is entered again for the next sub-frame.
The effective duration of the micro sleep mode between reception of two subsequent sub-frames is determined by the duration and the processing time of the PDCCH (which depend on the number of OFDM symbols to be received and demodulated), the time needed to switch the receiver component off and on, as well as inherent latencies (caused, e.g., by processing of the signal samples in the DFE). Accordingly, the effective duration Tmicro of the micro sleep mode can be expressed asTmicro=1 ms−(TDFE+TPDCCH+Tproc+Trxoff+Trxon)  Eq. (1),wherein TDFE is the DFE processing latency, TPDCCH is the duration of the PDCCH, Tproc is the processing time until the decision is available that there is no PDSCH scheduled in the current sub-frame, and Trxoff and Trxon are the periods of time it takes to switch the receiver components off and on, respectively. The micro sleep mode can only be entered if the effective duration Tmicro is larger than zero.
Assuming a normal (i.e., not extended) Cyclic Prefix (CP), a system bandwidth of more than 1.4 MHz (in which case the PDCCH is spread over a maximum of 3 OFDM symbols), and the following exemplary parametersTDFE=10 μsTPDCCH,max=215 μsTproc=250 μsTrxoff=15 μsTrxon=200 μs,the effective duration Tmicro of the micro sleep mode amounts toTmicro=1 ms−(10 μs+215 μs+250 μs+15 μs+200 μs)=310 μs
It should be noted that the above parameters are generally use case dependent, so there may be use cases in which the micro sleep mode can be entered and other use cases in which this may not be the case. Moreover, the processing time Tproc additionally depends on the available processing capabilities and the implemented channel estimation concept.
It has been found that micro sleep concepts of the type discussed above help to reduce the power consumed by the receiver. It would nonetheless be desirable to achieve a further reduction in receiver power consumption.